Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical methods. The iron-based particles are first molded in a die under high pressures in order to produce a desired shape. After the molding step, the structural component may undergo a sintering step to impart additional strength.
Research in the powder metallurgical manufacture of compacted components using iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties. Desired properties that often must be balanced include, for example, high density and strength, and ease of removing a part from a compacting die. Desirable properties for magnetic parts include, for example, a high permeability through an extended frequency range, high pressed strength, low core losses, and suitability for compression molding techniques.
Compaction of powder metallurgical compositions is carried out within a die cavity that is subjected to extreme pressures. To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. However, most known lubricants detrimentally affect the physical properties of compact parts. For example, use of lubricants often reduces the green strength of green compacts. It is believed that during compaction an internal lubricant is exuded between iron and/or alloying metal particles such that it fills the pore volume between the particles and interferes with particle-to-particle bonding. Indeed, some shapes cannot be pressed using known internal lubricants. Tall, thin-walled bushings, for example, require large amounts of internal lubricant to overcome die wall friction and reduce the required ejection force. Such levels of internal lubricant, however, typically reduce green strength to the point that the resulting compacts crumble upon ejection. Also, internal lubricants often adversely affect powder flow rate and apparent density, as well as green density of the compact, particularly at higher compaction pressures. Moreover, excessive amounts of internal lubricants can lead to compacts having poor dimensional integrity, and volatized lubricant can form soot on heating surfaces of the sintering furnace.
To avoid problems associated with internal lubricants, it is known to use an external spray lubricant rather than an internal lubricant. However, use of external lubricants increases compaction cycle time and leads to less uniform compaction. It is readily known to those skilled in the art that the inherent variability of using an external lubricant limits the commercial usefulness of such fabrication techniques. These limitations are especially prevalent in techniques for fabricating high density parts.
Accordingly, there exists a need in the art for methods of preparing high density compacted components that are easily ejected from die cavities.